1. Field of the Invention
The present invention relates to indication display units for vehicles, and more particularly to an indication display unit for vehicles which uses a so-called head-up display wherein an image indicated by an indicating unit is reflected on a reflecting surface such as an windshield, so that a virtual image is superposed on a front view outside therethrough.
2. Description of the Prior Art
Conventionally, there have been proposed various indication display units for vehicles in use of a head-up display as described above, wherein vehicle information such as vehicle speed, driving distance and the like are visibly observed by a driver without averting his eyes from a sight ahead during the driving of a vehicle.
However, generally, the driver's sight varies according to the velocity during the driving. For example, as shown in FIGS. 5A and 5B, the visibly observable range S at high speed (FIG. 5A) is narrower than the range S' at low speed (FIG. 5B).
Accordingly, there gives rise to a problem when a position of an indicated image, reflected on an windshield or the like, is set to a peripheral position of the visible range S' normally observed at low speed, the indicated image is greatly deviated from the visible range S normally observed at high speed, which causes a difficulty for observing the image. When a position of an indicating image is set to a peripheral position of the range S at high speed, the indicated image is to be positioned in the vicinity of a center of the range S' which can trouble the driver.
For solving the aforementioned problem, an indication display unit for vehicles has been proposed, wherein a projector for projecting an indicated image is rotated to vary a position of the indicated image according to the driving speed (Japanese Patent Laid-open No. 192912/1985).
FIG. 4 shows the above-described conventional indication display unit. A projector 2 has a casing 21 which encases therein an indicating unit 22, a filter 23 and a convex lens 24. An indicated image by the indicating unit 22 is projected on an windshield 1 through the filter 23 and the convex lens 24, and a virtual image X thereof is observed in superposition on an external front view through the windshield 1.
The projector 2 is mounted rotatably about a supporting member 26, wherein at low speed, force is applied from the direction such as indicated by arrow A, while at high speed, force is applied from the direction as indicated by arrow B so that a projecting position on the windshield is moved according to the vehicle speed.
However, according to the above-described conventional apparatus, a projector or the like is rotated to vary an image projecting position thereby an optical axis L directed to the projecting direction is radially moved substantially about one point. Therefore, as shown in FIG. 3, an incident angle .theta. with respect to the windshield 1 greatly varies as the optical axis L moves, and the reflecting direction thereof greatly varies in a direction of a or a' as viewed in FIG. 3.
Accordingly, there poses a problem in that the range (d in FIG. 3) in which the driver can visibly observe a displayed image without moving the driver's eye position is very narrow and the range in which the displayed image can be shifted is substantially narrow, and as a result the effect normally acquired by the shifting operation of the displayed position according to the vehicle speed as described above cannot be sufficiently obtained.